It is known in the art relating to catalytic converters to provide a housing of the clamshell type formed by two half shells joined along longitudinally extending flanges to form an enclosure for a catalytic element or substrate. In converters having frangible catalyst coated monolith elements, the housing may have a cylindrical interior of, for example, circular or oval cross section, depending on the shape of the enclosed element. A supporting mat may be provided to allow for expansion while holding the catalyst element firmly in place. The clamshell housings are easy to assemble but have a longitudinal seam along both sides which must be welded or otherwise sealed. Alternatively, a tubular or cylindrical body may be provided with separately formed end pieces that are welded on the ends of the tubular body. The end pieces include generally conical walls which connect the tubular portion enclosing the catalytic element with a smaller end opening for connection with an exhaust system pipe.